Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Richard Beck on 27/11/2010 19:30:03

Title: Why don't the stars in globular clusters crash into each other?
Post by: Richard Beck on 27/11/2010 19:30:03

Richard Beck asked the Naked Scientists:
   
Hi Ben,

I was listening today to one of your podcasts (http://www.thenakedscientists.com/HTML/podcasts/) talking about globular clusters (I'm catching up with them all after I found out that the NS are doing an astronomy version).  And I willed you on to ask my question, you got so close but didn't get there.

So here it is.

Globular clusters are a dense packet of stars packed in a small volume.  So much so that seeing stars in the middle is difficult.  They are also very old stars.  So after so many millions or billions of years why haven't these stars drifted together under gravitational attraction and formed a bigger and bigger central star or stars.   Indeed, the podcasts that I heard today said that stars such as our own drift away from the cluster.   With so many stars so close together why don't they crash into each other.  Are there any dopler shift figures for stars on opposite sides of a globular cluster?

Many thanks

Richard Beck

What do you think?

Title: Why don't the stars in globular clusters crash into each other?
Post by: Pikaia on 27/11/2010 19:45:59

In fact the stars do collide occasionally. Globular clusters contain stars called 'blue stragglers', which are stars that are rather massive, so they should have burned out if they were formed at the same time as the others. They are thought to be the result of two small, and therefore long-lived, stars colliding relatively recently.

Title: Why don't the stars in globular clusters crash into each other?
Post by: Soul Surfer on 28/11/2010 00:06:53

It is very difficult to understand how small stars are compared with the typical separation between them even in a very densely packed globular cluster.  Actual collisions between individual stars  almost never happen.  Most of the blue stragglers mentioned above were probably very close or contact binaries that merged as one star expanded towards a red giant phase and this allowed it to merge to become a single star with a stable but large angular momentum rather than a straight collision.

Title: Why don't the stars in globular clusters crash into each other?
Post by: Astrogazer on 02/12/2010 22:44:27

This question is to do with the gravitational attraction between the stars of a globular cluster. As the stars are so close to each other, there must be a "reasonable" attractive force between themselves and as they are old stars they have had millions or billions of years to move together.  So after so long, and they are not that far apart from each other, how is it that they can maintain their spacial separation?

Title: Why don't the stars in globular clusters crash into each other?
Post by: Soul Surfer on 02/12/2010 23:48:07

They are of course in orbits around the centre of gravity of the cluster.

Title: Why don't the stars in globular clusters crash into each other?
Post by: CPT ArkAngel on 03/12/2010 03:30:52

Clusters are relatively dense regions of space but they are still mostly composed of vacuum. All the stars and planets in clusters take much less than 1% of the space...

Title: Why don't the stars in globular clusters crash into each other?
Post by: yor_on on 04/12/2010 07:19:26

Maybe there exist some simulation on the net? We would need to know their common distances between each other, their mass and speeds relative each other too. Sound like someone already should have made a simulation I think.
I'll take a look later.

Title: Why don't the stars in globular clusters crash into each other?
Post by: yor_on on 05/12/2010 08:35:29

Okay, found two that one might try. The problem being that the simulations might be slow as there is a lot of stars involved, and a limited amount of computer capacity. Now, if they only could make those quantum computers work..

"GADGET is a freely available code for cosmological N-body/SPH simulations on massively parallel computers with distributed memory."

Gadet 2. (http://www.mpa-garching.mpg.de/galform/gadget/index.shtml)

Go and look, there are some nice movies there.

And.

N-Body. (http://www.ast.cam.ac.uk/~sverre/web/pages/nbody.htm)
Both are for Linux though, maybe there are some translation into Windows somewhere.

 

 

Title: Why don't the stars in globular clusters crash into each other?
Post by: Soul Surfer on 05/12/2010 19:09:31

CPT ArkAngel the figure is so much smaller than the 1% you quote I really must give you a more accurate figure.

A the core of a dense globular cluster can contain about 100 - 1000 stars per cubic parsec  say about 30 cubic light years that is a box abut 3 light years across.  This is compared with the one or less that is in our region of the galaxy.

Giving each star a cubic space about the size of the earth's orbit.  Thus is plenty of space to allow them to pass each other without any significant effects.

This box is around 20 light minutes across 3 boxes in the side of one light hour  24times for a day 365 for a year  say about 25,000  so there are   25,000x25,000x25,000 = 15,625,000,000,000 such boxes up to 1000 of which contain one star. so the odds that any box contains a star  is 1 in 15x10^9  and stars passing within these vast number of boxes are very unlikely to collide.

The sun is about 1.5 million Km across and the earth is 150  million Km from the sun so it represents about 1% of the length if the box to there are 100x100x100 = 1,000,000 boxes the size of the sun inside the earth's orbit.               

So if a star passes through a box the size of the earth's orbit the odds are around ten thousand to  one against a collision.

Considering that  typical velocities might be around 100km per second it takes one star about  150,000,000/100 seconds
this is around ten weeks or 1/5 of a year

Putting this all together there is a 1 in 10,000 chance of a collision 15/5 x 10^9 years  that is one direct collision every thirty  trillion years  agreed there are a lot of globular clusters in the universe but collisions are vastly rarer than supernovae